Graded gap semiconductor optical device

ABSTRACT

A variable temperature method for the preparation of single and multiple  taxial layers of single-phase (e.g., face-centered cubic), ternary lead chalcogenide alloys (e.g., lead cadmium sulfide [Pb 1-w  Cd w  ] a  [S] 1-a  wherein w varies between zero and fifteen hundredths, inclusive, and a=0.500±0.003), deposited upon substrates of barium fluoride, BaF 2 , maintained in near thermodynamic equilibrium with concurrently sublimated lead alloy and chalcogenide sources. During preparation, the temperature of the substrate is varied, thereby providing an epilayer with graded composition and predetermined electrical and optical properties along the direction of growth. This growth technique can be used to produce infrared lenses, narrowband detectors, and double heterojunction lasers.

This is a division of application Ser. No. 864,417 filed Dec. 27, 1977now U.S. Pat. No. 4,227,948.

BACKGROUND OF THE INVENTION

This invention generally relates to a method of preparing epitaxialfilms for use as infrared detectors, lenses, and light emission devices,and more particularly, to lead chalcogenide epitaxial films suitable forelectro-optical applications. Additionally, this invention relates to asimplified method of controlling the composition or energy gap of thesemiconductor material during epitaxial growth.

Thin-films of lead salt alloys have been investigated intensivelyrecently with particular attention to their photovoltaic properties.Special attention has been paid to their possible use as detectors ofinfrared radiation.

The exploration of vacuum deposition techniques is quite recent and, forthe benefit of those who may not be familiar with the pioneering effortsin this art, the following brief bibliography is made of record.

Preparation of Single-Crystal Films of PbS, by R. B. Schoolar and J. N.Zemel, Journal of Applied Physics, Vol. 35, No. 6, (June, 1964), pp.1848 to 1851.

Epitaxial Lead-Containing Photoconductive Materials, by R. B. Schoolar,H. R. Riedl, and J. P. Davis, U.S. Pat. No. 3,574,140, (April 1971).

Method of preparation of Lead Sulfide PN Junction Diodes, by R. B.Schoolar, U.S. Pat. No. 3,716,424, (February 1973).

Method of Varying the Carrier Concentration of Lead-Tin SulfideEpitaxial Films, by R. B. Schoolar, U.S. Pat. No. 3,793,070, (February1974).

Photoconductive PbSe Epitaxial Films, by R. B. Schoolar and R. J.Lowney, Journal of Vacuum Science Technology, Vol. 8, No. 1, (1971).

Surface Charge Transport In PbS_(x) Se_(1-x) And Pb_(1-y) Sn_(y) SeEpitaxial Films, By J. D. Jensen and R. B. Schoolar, Journal of VacuumScience Technology, Vol. 13, No. 4, (1976).

More recent efforts, although originating from a different direction,include:

Properties of PbS_(1-x) Se_(x) Epilayers Deposited Onto PbS SubstratesBy Hot-Wall Epitaxy, by K. Duh and H. Preier, Journal of Vacuum ScienceTechnology, Vol. 10, pp. 1360, (1975).

PbSe Heteroepitaxy By The Hot-Wall Technique, by K. Duh and H. Preier,Thin Solid Films, Vol. 27, p. 247, (1975).

Double Heterojunction PbS-PbS_(1-x) Se_(x) -PbS Laser Diodes With CWOperation Up to 96K, by H. Preier, M. Bleicher, W. Riedl, and H. Maier,Applied Physics Letters, Vol. 28, No. 11, (June, 1976).

PbTe and Pb₀.8 Sn₀.2 Te Epitaxial Films On Cleaved BaF₂ SubstratesPrepared By A Modified Hot-Wall Technique, by T. Kasai, D. W. Bassett,and J. Hornung, Journal of Applied Physics, Vol. 47, (July, 1976).

Double-Heterostructure PbS-PbSe-PbS Lasers With CW Operation Up to 120K,by H. Preier, M. Bleicher, W. Riedl, and H. Maier, Journal of AppliedPhysics, Vol. 47, (December 1976); and,

Growth of PbTe Films Under Near-Equilibrium, by A. Lopez-Otero, Journalof Applied Physics, Vol. 48, No. 1, January 1977.

It is well established that single crystal films of PbS, PbSe, andrelated compounds, hereinafter referred to as lead salt alloys, can beepitaxially grown on heated alkali halide substrates by vacuumevaporation. It is also known that the conductivity type of thesesemiconductors in bulk form can be controlled by regulating deviationfrom stoichiometry. Anion (lead) vacancies make these crystals P-typeand cation vacancies make them N-type.

In the past, films of the lead salts have been produced through the useof various deposition techniques. These films were homogeneous withconstant energy gaps both across their length and throughout theirthickness.

SUMMARY OF THE INVENTION

In U.S. Pat. No. 4,154,631 entitled Equilibrium Growth Technique ForPreparing PbS_(x) Se_(1-x) Epilayers, filed on the 27th day of May,1977, R. B. Schoolar, a co-inventor herein, disclosed a novel processwhereby a single-phase lead sulfide selenide, [Pb]_(a) [S_(x) Se_(1-x)]_(1-a), epilayer, where x varies between zero and one, inclusive, anda=0.500±0.003, with predetermined electrical and optical properties isprepared by an equilibrium growth technique (EGT). An alkali-halidesubstrate maintained in near thermodynamic equilibrium with the sourcecharges is exposed to the single chimney orifice of a two-zone,dual-chamber furnace in which a homogeneous vapor has been produced byconcurrent sublimation of a lead alloy in one chamber and a measuredamount of chalcogenide in an appendant chamber. Regulation of thecomposition of the lead alloy charge controls the energy gap and thusthe spectral response of the sublimate. Regulation of the ratio betweenthe metal alloy and chalcogenide vapors controls deviations fromstoichiometry in the sublimate and thus its conductivity type andcarrier concentration. A substitution of materials allows single-phase,epilayers of Pb_(1-y) Sn_(y) Se, 0≦y≦1.0, to be prepared by thistechnique. By periodically varying the temperature of the chalcogenidein the appended furnace from below to above its sublimation temperature,multiple planar junction films may be prepared as successive epilayersof the film which will exhibit opposite type conductivities.

The present invention, a modification of the equilibrium growthtechnique, lies in the step of varying the alloy composition of a leadchalcogenide epilayer during its growth by changing the temperature ofthe heated substrate. The resulting single-phase epilayer isface-centered cubic with a graded composition through its thickness and,therefore, exhibits a graded energy gap along the same dimension.Incorporation of this step into the equilibrium growth technique permitsgrowth of device quality Pb_(1-w) Cd_(w) S thin-film epitaxial layers(i.e., "epilayers") where 0≦w≦0.15, from a single ingot of lead-cadmiumsulfide with w≧0.15.

Accordingly, one object of the invention is to provide a method forepitaxially preparing thin-films of lead-cadmium sulfide.

A second object of the invention is to provide a method for epitaxiallypreparing thin-films of lead cadmium sulfide having a compositiondifferent than that of the source material.

Another object of the invention is to provide a method for epitaxiallygrowing thin-films of ternary lead salt alloys.

Yet another object of the invention is to provide a method forcontrolling the composition of ternary lead salt alloy films duringgrowth.

Still another object of the invention is to provide a method forepitaxially growing ternary lead salt alloy thin-films with predestinedvariations in composition across their thicknesses.

Yet another object of the invention is to provide a method forepitaxially growing a ternary lead salt alloy thin-film in which thealloy composition of the film is varied during growth.

A further object of the invention is to provide a ternary leadchalcogenide epitaxial film of quality sufficient for photovoltaicapplications.

A yet further object of the invention is to provide a ternary lead saltalloy epitaxial film having a composition tuned spectral response.

A still further object of the invention is to provide a ternary leadchalcogenide thin-film of smooth stoichiometry across its width,suitable for photovoltaic applications.

An additional object of the invention is to provide a ternary leadchalcogenide thin-film of uniform conductivity across its width,suitable for photovoltaic application.

Another object of the invention is to provide a lead-cadmium sulfidephotovoltaic detector.

Still another object of the invention is to provide a process foraltering the alloy composition of a lead salt thin-film during growthwithout changing to a second growth apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of this invention, and many of theattendant advantages thereof, will be readily appreciated as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawings in which like numbers indicate the same or similar components,wherein:

FIG. 1 is a cross-sectional diagram of a sublimation apparatus in whichthe process of this invention may be practiced.

FIG. 2 is a temperature profile range during growth along the length ofthe apparatus shown in FIG. 1. A logarithmic temperature scale isarrayed along the abscissia.

FIG. 3 is an orthogonal projection of a photovoltaic detector preparedaccording to the equilibrium growth technique.

FIG. 4 is a graph illustrating the variation in mole fraction ofcadmium-sulfide in an epitaxial layer as a function of the substratetemperature during growth.

FIG. 5 is an orthogonal graph illustrating the Hall mobility at 77° K.for twenty-eight Pb_(1-w) Cd_(w) S, films as a function of substratetemperature during growth.

FIG. 6 is an orthogonal graph illustrating the Hall coefficient at 77°K. as a function of substrate temperature during growth of twenty-eightlead-cadmium sulfide samples.

FIG. 7 is an orthogonal graph showing the dependence of the Hallcoefficient at 77° K. as a function of thickness in various Pb_(1-w)Cd_(w) S epilayers with the substrate temperature during growth as aparameter.

FIG. 8 is a graph of Hall mobility versus substrate temperature forseveral substrate materials.

FIG. 8A is a cross-sectional view of a single layer narrowband detector.

FIG. 9A is an orthogonal graph illustrating the change in the energy gapas a function of the thickness of the film for three devices preparedaccording to the present invention.

FIG. 9B is an orthogonal graph illustrating the change in the energy gapas a function of the width of the film for the three embodimentsrepresented by the curves of FIG. 9A.

FIG. 10 is a cross-sectional view of a heterojunction laser preparedaccording to the present invention.

FIG. 11A is an orthogonal graph illustrating the variation in the energygap as a function of thickness of adjoining layers of the film shown inFIG. 10.

FIG. 11B is an orthogonal graph illustrating the variation in the energygap as a function of width of layers of the film shown in FIG. 10.

FIG. 12A is a side view of a point contact substrate heater.

FIG. 12B is an orthogonal graph illustrating the variation in substratetemperature produced with the heater shown in FIG. 12A, in units of onethousand degrees Kelvin, as a function of substrate width.

FIG. 13A is an oblique view of a circumferential substrate heater.

FIG. 13B is an orthogonal graph illustrating the variation in substratetemperature produced with the heater shown in FIG. 13A, in units of onethousand degrees Kelvin, as a function of substrate width.

FIG. 14 is a cross-sectional view of a thin-film lens prepared accordingto the present invention.

FIG. 15A is an orthogonal graph illustrating the variation in the energygap as a function of the thickness of the film for two embodiments ofthe thin-film lens shown in FIG. 14.

FIG. 15B is an orthogonal graph illustrating the variation in the energygap as a function of the width of the film for the two embodimentsrepresented by the curves of FIG. 15A.

DETAILED DESCRIPTION

Referring now to the drawings, and in particular to FIG. 1, there isshown a sectional view of a two-temperature zone vacuum depositionapparatus dedicated to the preparation of device quality epilayers oflead salt alloys by equilibrium growth techniques. The apparatus shownis a conventional glass belljar system with a nitrogen cold trap and oildiffusion pump 54, mounted upon a base 50. The central component of theapparatus illustrated is a two-zone, dual-chamber furnace knowncolloquially as an "insert." A vertical primary furnace 20, typicallyfabricated from quartz glass, discharges through its upper extremityinto a single orifice chimney 26 of wider diameter. A smaller vertical,secondary furnace 30, also of quartz glass, known as a "cold tip", isformed by a coaxially appended (i.e., co-axially to assure a moreuniform distribution of sublimate), blind tube extending through thebottom, and discharging into, above the contents, of primary furnace 20.A simple metal stand (not shown) holds the insert upright. Duringoperation, the contents 2, 8, of primary and secondary furnaces 20, 30are raised to, and maintained at, a desired temperature by a pair ofindependently controlled, external thermal devices 22, 32, shownhelically coiled around the lower extremities of the primary andsecondary furnaces 20, 30. Typically, thermal device 22 is a heater,such as a nichrome wire coupled to a current regulated power supply.Convection and radiant heating occurs between the primary and the upperthird of the secondary furnace 20, 30. Depending upon the contents ofsecondary furnace 30, thermal device 32 may be either a heating coillike thermal coil 22, or a cooling coil. If the chalcogenide charge 8 issulfur for, example, thermal device 32 may be a short length of pipe ortube placed against base 50 and carrying tap water flowing at a ratesufficient to partially negate convection heating and maintain thesulfur charge 8 at a lower temperature than the metal alloy charge 2 inthe primary furnace 20. A glass baffle 24 located between oppositeextremities in primary furnace 20 restricts the size of the moleculesentering chimney 26.

Placed directly above chimney 26 are one or more substrate heaters 40,each of which raise to, and maintain at, a desired temperature aretensively held substrate layer 12 with one face exposed to the orificeof chimney 26. A commercially available thickness monitor 60 is, spacepermitting, located above a port 44 in substrate holder 40 so that itsreplaceable crystal face 62 is exposed to the sublimate passing throughport 44. A mask and shutter mechanism 42 located between the orifice ofchimney 26 and the exposed faces of substrate layers 12, is normallyclosed to prevent condensation of the charge materials when the furnacesare not at the proper temperatures. Thermocouples 64, 66, 68 monitor thetemperature of source charge 2, chalcogenide 8, and substrates 12,respectively. An ion gauge 52 is used to measure the vacuum insidechamber 56 below 10⁻⁴ Torr. The metal alloy source charge 2 is preparedby mixing weighted masses of research grade component elements of thedesired alloys, [Pb_(1-w) Cd_(w) ]_(a) [S]_(1-a), where a=0.500±0.003,in proportion to their atomic weight in the composition of the desiredepilayer, heating the mixture in an evacuated chamber until it forms anall-reacted melt, and cooling the melt to room temperature. Theresulting polycrystal is pulverized into course granules. The atomicfraction, a, is varied within ±0.003 of a stoichiometric mixture to makethe mixed mass either slightly metal or chalcogenide rich, as desired. Ametal rich mixture, a>0.500, will produce an N-type conductivityepilayer in the absence of a compensating chalcogenide dopant 8, while achalcogenide rich mixture, a<0.500, will produce a P-type conductivityepilayer.

Dopant 8 is a six nines (i.e., 99.9999%) pure chalcogenide (e.g., S,Se). The substrates 12 are freshly air-cleaved or polished slices of asingle crystal of any orientation such as BaF₂, SrF₂, NaCl, KBr, or anyof the lead salt alloys. The substrates can either be insulating orconducting. The bottom of primary furnace 20 is charged with perhapstwenty grams of the pulverized granules of a metal alloy 2, such as[Pb_(1-w) Cd_(w) ]_(a) [S]_(1-a), where 0≦w≦0.15. A twenty gram charge 2is enough to prepare approximately twenty epilayers. The bottom of thecold tip, secondary furnace 30, is charged with perhaps one gram ofchalcogenide dopant 8.

Prior to operation, prepared crystalline substrates 12 are placed insubstrate heater 40. Chamber 56 is evacuated to a pressure on the orderof 10⁻⁶ Torr (i.e., approximately 1.3·10⁻⁴ Pa), although only a vacuumbetter than 10⁻⁵ Torr is necessary. The temperature of substrates 12 israised to and maintained within the 325° to 515° C. range. Thestoichiometry "a" of epilayer 14, in FIG. 3, is independent of thesubstrate temperature. The metal alloy charge 2 in primary furnace 20 israised to between 550° and 650° C. and is maintained within ±5° C. ofthis range. The temperature of dopant charge 8, if the dopant is sulfur,is maintained at room temperature with a tolerance of ±5° C. Prior tosublimation, a substrate temperature is empirically chosen within thisrange to allow epitaxial growth. Changing the temperature of either, orboth, metal alloy charge 2 or substrate 12 from the lower end of theirrespective temperature ranges up to approximately 410° C. merely changesthe rate of growth of the epilayer film 14 within a range between 2 and20 microns per hour. The quality of the film is not sensitive to therate of growth within these ranges. The thickness of the films grown maybe varied from approximately one hundred angstroms to well beyond fiftymicrons.

Turning now to FIG. 2 of the drawings, a vertical operating temperatureprofile of the two temperature zone apparatus of FIG. 1 is set forth inrectangular coordinates for preparation of a Pb_(1-w) Cd_(w) S film. Alogarithmic temperature scale from approximately room temperaturethrough one thousand degrees Kelvin is arranged along the abscissa, anda half profile of part of the apparatus is arranged along the rightordinate with a linear scale in centimeters along the left ordinate. Asshown, the operating temperature inside the apparatus varies from 300°K. (26° C., ±5°), through the sulfur dopant charge 8, to the 770° to925° K. (500° to 650° C.) range through the metal alloy charge, to the598° to 790° K. (325° to 515° C.) range at the substrates 12. Withchamber 56 evacuated to a gauge pressure below 10⁻⁶ Torr and dopantcharge 8 at room temperature, the vapor pressure in the primary furnaceis controlled by the vapor-temperature of the sulfur dopant charge,approximately 2×10⁻⁴ Torr. Thermodynamic equilibrium may be defined (byignoring the Gibbs function) as a condition in which an isothermalprofile exists throughout the length of the growth chamber of theapparatus. The deviation of the profile shown in FIG. 2 from anisothermal assures vapor transport of molecules 6 as well as theircondensation upon substrates 12. The necessity of maintaining thetemperatures of only two areas of the growth chamber--the main furnacecharge 2 and the substrates 12--within specific ranges provides thesimplicity of this near equilibrium growth technique and its associatedapparatus.

Once the selected operating pressure and temperatures are reached,sublimation of the source charges 2, 8 occurs, shutter mechanism 42 isopened, and those molecular particles 4 and 10 rising past baffle 24(i.e., molecular particles 6) condense upon the exposed surface of eachsubstrate 12. Baffle 24 is so placed that the larger of particles 4, 10cannot pass, and that no particle can travel from a source charge 2, 8to substrate 12 in a straight line. Without baffle 24, large particleswould quickly rise through chimney 26, travel to the surface ofsubstrate 12, and either stick and shadow the adjacent surface area,thereby causing pin-holes in the epilayer or hit and richochet from thesurface, thereby possibly fracturing epilayer 14. Some particles 6 passthrough port 44 and condense upon the crystal face 62 or thicknessmonitor 60. This condensation decreases the resonant frequency ofpiezoelectric crystal 62, thereby enabling the amount and rate of growthof the epilayer on nearby substrate 12 to be accurately observed. Whenan epilayer 14 of the desired thickness is grown, shutter 42 is closed,substrate 40 and furnace 22, and, if used 32, heaters are shut off, andatmospheric pressure is restored to the chamber. After cooling to roomtemperature, the lead chalcogenide epilayer 14 is exposed to atmosphereand placed into a second evaporator where a Schottky barrier planarjunction 116, as shown in FIG. 3, is formed by vacuum depositing on theexposed face of film 14 through a stainless steel mask either a centraldot or strip of metal (e.g., indium, bismuth, lead or tin) 100. Theas-grown film is vacuum annealed at 170° C. for thirty minutes, and thencooled to room temperature prior to depositing the lead Schottkybarriers. This step desorbs oxygen from the air exposed surface and istaken to assure production of a stable device. None of the samples weresurface passivated in the following Examples. It should be noted thatexposure of the fresh films to atmosphere without passivation causesoxidation of their surfaces with a concomitant reduction in theirelectrical characteristics. In one experiment, the mask was a narrowarray of circular openings with areas of 3.2 ×10⁻³ cm² on 0.127 cmcenter spacing. The metal dot 100 inverts the semiconductor and convertspart of the underlying volume (i.e, region 117) of epilayer 14 into aregion 117 of N-type conductivity. The deposited metal dot 100 serves asan electrical contact on which an electrical lead 112 to an externalcircuit may be attached with lead solder, or gold or silver paint 101.After depositing the barriers, the epilayers are exposed to atmosphereand inserted into a third vacuum system where a thin metallic layer(e.g., gold or platinum) is deposited elsewhere on the exposed face ofepilayer 14 to form an ohmic electrode 120. The gold pad 120 was usedfor resistivity and Hall coefficient measurements, and served as ohmiccontacts for the detector measurements. An ohmic electrode is one thatsupplies a reservoir of charge carriers that are freely available toenter epilayer 14 as needed. The detector samples were then mounted inan optical dewar with a twenty degree field-of-view and the threeSchottky barriers on each film were evaluated. Typically, the impingentradiation 1 passes through substrate 12 and enters epilayer 14.Therefore the material chosen for substrate 12 must be transparent tothe wavelengths of interest. BaF₂ and SrF₂ become opaque atapperoximately 14 microns and KBr at approximately 37 microns. Bydepositing a very thin (e.g., on the order of 100 A) layer of metal toform electrical contact 100, the contact will be semi-transparent in thenear, intermediate and far infrared bands, and the detector may bedisplayed to allow impingent radiation 1 to enter epilayer 14 withoutfirst passing through substrate 12.

It is apparent that the composition of an epilayer prepared according tothe teachings of the foregoing description is dependent upon thecomposition of the source material. A proposition of this disclosurehowever, is that the composition of the asgrown epilayer need not beidentical to the composition of the source charge. In particular, byvarying, rather than maintaining the temperature of the substrate 12within the stated ranges, the mole fraction of the epilayer may bechanged during growth.

The general nature of the invention having been set forth, the followingillustrative example is presented as a typical embodiment thereof. Itwill be understood that the invention is not limited to this example,but is susceptible to various modifications that will be recognized byone of ordinary skill in the arts. The true values of the charge carriermobilities given in the example are obtained after taking the oxidizedsurface layer into account by the Petritz two layer analysis, as setforth in "Surface Charge Transport In PbS_(x) Se_(1-x) And Pb_(l-y)Sn_(y) Se Epitaxial Films," by J. D. Jensen and R. B. Schoolar, Journalof Vacuum Science Technology, Vol. 13, No. 4, 1976.

EXAMPLE I

The apparatus shown in FIG. 1 has been used to deposit epitaxial filmsof Pb_(1-w) Cd_(w) S (0≦w≦0.15) on freshly cleaved (111) BaF₂.Deposition pressures were on the order of 10⁻⁶ Torr (i.e., ˜1.3×10⁻⁴ Pa)at the gauge 52 and the substrate temperatures were varied between 400°and 513° C. The main furnace 20 was maintained at 565° C. Growth rateswere in the range of two to four microns per hour. The distance betweensource 2 and substrate 12 was three and one-half centimeters, and theprimary furnace 20 was two centimeters in diameter. Approximately twentygrams of granulated, 0.003 metal rich Pb₀.94 Cd₀.06 S source material 2was placed in the upper furnace 20. This was a charge of materialsufficient to obtain fifteen to twenty films of constant composition.The coaxial auxiliary or secondary furnace 30 (used to compensate forthe loss of the more volatile element) was charged with a small amountof pure sulfur 8. The sulfur source was used to control the carrier typeand concentration of the films when needed. When down to pressure, thesubstrate heater was turned on and allowed to come to a steady statetemperature between 325° C. and the approximate sublimation temperatureof the source material, 510° C. The primary oven was then heated tobetween 565° and 650° C., the shutter opened after thermal equilibriumwas obtained, and the films were deposited onto the BaF₂ substrates.After growth the films were cooled to room temperature, and removed fromthe evaporator. Gold pads were then deposited onto the films fortransport measurements.

The transport properties at 77° K. of seven films of Pb_(1-w) Cd_(w) Sgrown at different substrate temperatures, T_(s), with w varying between0.002 and 0.060 are set forth in Table 1. The variation in compositionof seven of the epilayers prepared is shown in the graph of FIG. 4 wherethe mole fraction, w, of cadmium is plotted as a function of substratetemperature during growth. The epilayers were grown from a singlePb_(1-w) Cd_(w) S source charge, w=0.06. By maintaining the substratetemperature at selected values between 515° and 400° C., the molefraction, w, of cadium-sulfide on successively grown epilayers wasvaried inversely with substrate temperature between 0≦w≦0.06. Duringgrowth, the substrate temperatures were maintained within ±5° C. of thetemperatures indicated.

The foregoing detailed discussion describes a growth technique forpreparing device quality, single-phase, lead-cadmium sulfide epilayersof controlled composition and energy gap. The technique allows theenergy gap of the epilayer to be varied uniformly with thickness bychanging the temperature of the substrate during growth therebyproviding a process for preparing composition tuned detectors within thespectral region from two to four microns. Similarly, by varying thecomposition of other lead chalcogenide alloy charges, this processallows preparation of composition tuned detectors from the near to thefar-infrared regions. In addition, the minority charge carrier lifetimesof epilayers prepared under the teachings of the present process arelengthened and the number of crystal defects reduced, by

                  TABLE I                                                         ______________________________________                                        PROPERTIES OF Pb.sub.1-w Cd.sub.w S FILMS                                             THICK-                                                                        NESS     N        .sup.μ H                                                                           E.sub.g   T.sub.s                           SAMPLE  μm    10.sup.17 cm.sup.-3                                                                    cm.sup.2 V.sup.-1 S.sup.-1                                                            eV   w    °C.                        ______________________________________                                        326     1.83     1.7      340     0.504                                                                              0.058                                                                              408                               330     1.53     2.1      420     0.477                                                                              0.050                                                                              440                               331     1.72     3.0      820     0.407                                                                              0.029                                                                              440                               332     3.20     5.2      160     0.517                                                                              0.060                                                                              408                               359     2.35     2.8      8800    0.318                                                                              0.002                                                                              513                               396     0.90     6.3      2200    0.413                                                                              0.031                                                                              483                               400     2.72     2.3      3000    0.370                                                                              0.015                                                                              483                               ______________________________________                                    

maintaining an elevated substrate temperature during growth, therebyallowing for room temperature operation of the detectors prepared fromthe epilayers.

Hall mobility measurements made at 77° K. are plotted in FIG. 5 as afunction of the substrate temperature during growth for a series ofP-type lead-cadmium sulfide epilayers. The mobilities are very low forsamples grown upon substrates maintained at less than 400° C. duringdeposition. The maximum hole mobility at 77 K. is 8,800 cm² V⁻¹ sec⁻¹,and occurs in an epilayer grown upon a substrate maintained at 513° C.Mobility degraded in epilayers grown upon substrates maintained attemperatures greater than 513° C. due to thermal etching of the sampleand substrate. The Hall coefficients at 77° K. are plotted in the graphof FIG. 6 as a function of substrate temperature during growth for thesame P-type lead-cadmium sulfide epilayers. Those epilayers grown uponthe hotter substrates tended to have lower hole concentrations. Itshould be noted that some of the scatter in the data is due tovariations in the epilayer thickness. This dependence is shown by thegraph of FIG. 7 where the Hall coefficient at 77 ° K. is plotted as afunction of sample thickness for two growth temperatures. One set,indicated by the square data points representing epilayers grown uponsubstrates maintained at 440° C., shows slight thickness dependence. Theother set, indicated by triangular data points representing epilayersgrown upon substrates maintained at 480° C., clearly shows thedependence. Samples grown upon substrates maintained at lowertemperatures (i.e., less than 350° C.) show no dependence by the Hallcoefficient upon epilayer thickness.

Diverse modifications to the process disclosed in the foregoingteachings are possible. For example, by changing the temperature of thesubstrate within the range between 350° C. and the sublimationtemperature of the alloy source, the cadmium mole fraction of thesubsequent film growth will differ from that of the preceding layer. Asthe adjoining layers differ in composition, they will exhibit peakresponsivities at different wave lengths. Periodic variation of thesubstrate temperature will provide an epilayer with adjoining, distinctlayers of different composition. Typically, the source alloy isdescribed in these teachings as lead-cadmium sulfide. Without deviatingfrom the principles of the process, either lead-cadmium selenide orlead-cadmium telluride may be used as a source alloy.

In a further modification, selenium could be substituted for the sulfurdopant charge. Since selenium has a much lower vapor pressure, the "coldtip" 30 would have to be heated to approximately 130° C. to obtainsingle-phase films. Higher tip temperatures would produce P-type filmsand lower tip temperatures would yield N-type layers. Similarly,tellurium could be used as a dopant charge. Furthermore, this techniqueprovides a method to prepare either single or multiple planar junctiondevices. By periodically varying the temperature of the chalcogenidecontents in cold tip 8 from below to, or above, its sublimationtemperature, the partial pressure of the chalcogenide vapor in uniformlymixed molecular particles 6 will increase, resulting in a change in theconductivity of the subsequent thickness of the film. If alloy charge 2is slightly metal rich, and during the initial growth of epilayer 14 thetemperature of the chalcogenide 8 in cold tip 30 is kept below thesublimation temperature of the chalcogenide, the first growth willexhibit a N-type conductivity. If the temperature of chalcogenide israised to, or above, the sublimation temperature of chalcogenide, thesubsequent growth of epilayer 14 will exhibit a P-type conductivity.Returning the temperature of chalcogenide 8 to below its sublimationtemperature will cause the next growth to exhibit N-type conductivity.As the process assures a uniform mixture of the vapors from metal alloy2 and chalcogenide 8, and thus growth of uniform stoichiometry,successive layers will clearly define a planar P-N junction.

The simplicity of the graded-gap growth technique disclosed in theforegoing paragraphs facilitates preparation of numerous semiconductordevices. Refer now to FIG. 8 where a cross-sectional view illustratesthe structure of a back-illuminated, narrowband detector preparedaccording to the teachings of the present invention by growing a singlelayer 14 of a P-type semiconducting material upon a transparent,insulting substrate 12 (not shown in FIG. 8). Typically, surface 200adjoins the substrate 12. A shallow P-N junction 117 is formed byevaporation of a lead contact 100 upon surface 300. The lead completelyinverts the semiconductor epilayer 14 beneath contact 100 to N-typeconductivity. Ohmic contact 101 is created by evaporation of a gold orplatinum contact onto part of the remaining surface 300 of epilayer 14.All of these processes are completed in a vacuum of at least 10⁻⁶ Torrat the gauge.

As is more fully taught in the copending application of R. B. Schoolarfor U.S. Letters Patent, entitled Narrowband Infrared Detector, assignedSer. No. 833,798 filed in the Patent Office on the 16th of September,1977, (now abandoned), the energy gap, E_(F), of the region of layer 14exposed to incident radiation 1 (i.e., the "filter" region) must equalor exceed the energy gap, E_(D), of the region on the opposite side ofthe layer from the exposed region (i.e., the "detector" region):

    E.sub.F ≧E.sub.D.                                   (1)

FIG. 9A shows three curves representing three epilayers in which thevariation in the energy gap of layer 14 as a function of its thickness,d is an ordered continuum. Curve D1 represents a narrowband detector inwhich E_(F) equals E_(D), a relation obtained by maintaining thetemperature of the substrate constant during growth of layer 14. CurvesD2 and D3 represent narrowband detectors in which the energy gaps acrossthe filter regions exceeds the energy gap across the detector regions, arelation obtained by varying the temperature of the substrate duringgrowth either linearly or nonlinearly respectively, with respect to therate of growth. As indicated by the three curves of FIG. 9B (curves W1,W2, and W3 correspond to the devices represented by curves D1, D2, andD3, respectively), the energy gap across any plane parallel to surfaces200, 300 (i.e., the "width-of-growth" of layer 14) is constant.

It is quite easy, using the graded-gap technique, to prepare anarrowband detector having adjoining filter and detector regions, eachwith constant compositions (and thus, energy gaps) across theirthicknesses, but with the two energy gaps changing abruptly at thejunction between the two regions. During preparation of such a device,after growth of a filter region having the desired thickness, the growthis interrupted by closing the shutter mechanism 42, the temperature ofsubstrate 12 is raised, and after the reoccurrence of equilibriumconditions, shutter mechanism 42 is reopened. The step function curve D4in FIG. 9A is an example of the energy gap profile across the thicknessof a narrowband detector prepared in this manner.

FIG. 10 shows a side view of a heterojunction laser prepared accordingto the teachings of the presently disclosed process as a three layer,graded-gap device continuously grown upon a substrate (not shown)adjacent to either layer 141 or 143. As is shown by curves of FIG. 11A,the energy gap varies abruptly between surfaces 200, 300. The outerregions 141, 143 have substantially constant energy gaps, as isindicated by curves D6, each of which are higher than that of theinterior region 142 of the layer 14. By closing the shutter mechanism 42and increasing the temperature of substrate 12 (not shown in FIG. 10)during the growth process, and then reopening shutter 42 after thereoccurrance of equilibrium conditons, the composition, but not the typeof conductivity, of the subsequently grown thickness of the layer isabruptly altered, producing a region 142 having a lower energy gap, asis indicated by curve D6, than that of the previously grown region 141.After region 142 obtains the desired thickness, again closing shutter42, decreasing the temperature of substrate 12, and then reopeningshutter 42 after reoccurrence of equilibrium conditions will provide asubsequently grown third region 143 having a composition, and thus anenergy gap, abruptly differing from that of the previously grown region142, as is indicated by curve D5. FIG. 11B shows that the energy gaps ofregion 142, as indicated by curve W6, and regions 141, 143, as indicatedby curve W5, are substantially constant across the width-of-growth(i.e., between surfaces 202 and 204). The abrupt changes in the levelsof the energy gaps D5, D6 at the junctions between regions 141, 142 and142, 143 distinctly define the active region 142 and the two junctions.To function as a hererojunction laser, one of the two outer regions 141,143 must differ in stoichiometry, composition and type of conductivityfrom the active region 142. If regions 143, 142 are grown with a P-typeconductivity, then region 141 must be grown with an N-type conductivity.Electrical conductors 101, 103 are placed on any portion of the outersurfaces 300, 200 respectively, of regions 141, 143. Application of asignal to terminals 131, 133 causes an injection current to flow throughthe entire layer 14, resulting in laser action supported by the forwardbiased of the P-N junction 141-142. The surfaces formed by 141-142 andthe opposite junction, 142-143, act as optically confining surfaces.Since there is a gain in the plane of the forward biased junction, thedevice will emit a laser beam 1' along the plane between naturallyreflective surfaces 202 and 204. A primary advantage of the double layerheterojunction laser is its high efficiency. The advantage of our devicelies in the simplicity of its production. The laser is growncontinuously without breaking vacuum. The junctions are therefore notexposed to air during growth.

The details of the preceding paragraphs describe the instant process asone for preparing semiconductors having an energy gap graded across thedirection of growth (i.e., the "thickness of growth"). In this process,the semiconductor is grown upon a substrate that typically, at any giveninstant, is uniformily maintained at a selected temperature. The energygap of the semiconductor may be either smoothly or abruptly gradedacross its thickness by either gradually varying the substate tempertureor by closing the shutter mechanism to interrupt the growth process,changing the substrate temperature, and then, upon reoccurrence of near(as determined by monitoring the thermocouples 64, 66, 68) equilibriumconditions, reopening the shutter. A modification of this process allowspreparation of a semiconductor having an energy gap graded along theplane normal to the direction of growth (i.e., along its"width-of-growth"). The modification requires maintenance of atemperature profile along the exposed face of the substrate duringgrowth of the semiconductor. FIG. 12A is a side view of a substrate 12indirectly heated by a point contact heater element 40 (i.e., a heat"source"). A metal mounting plate 41 (i.e., a heat "drain") separatesthe face opposite the exposed face of substrate 12 from, and distributesthe heat of, heating element 40. Consequently, the exposed face ofsubstrate 12 exhibits a parabolic temperature profile. FIG. 12B showssuch a temperature profile with curve r1.

FIG. 13A is an oblique view of a substrate 12, cleaved in the shape of awafer, indirectly heated by a spiral nichrome heater element 40'. Ametal mounting plate 41' (i.e., a heat "drain") separates the faceopposite the exposed face of substrate 12 from, and distributes the heatsupplied by, heater element 40'. Curve r2 in FIG. 13B shows thetemperature profile of substrate 12 along its face.

FIG. 14 is a cross-section of a back illuminated semiconductormonocrystalline or polycrystalline layer 14 grown according to themodification of the graded gap process upon an unequally heatedsubstrate. If deposited upon a substrate in which the same unequaltemperature profile is maintained during growth, the layer 14 will be anoptically flat film with a constant energy gap, E_(g), along any lineextending through its thickness (e.g., curves D7, D8 in FIG. 15A) butwith a varying energy gap along its width (e.g., curves W7, W8 of FIG.15B). If prepared using the center-of-substrate heating apparatus ofFIG. 12A, a layer 14 grown from a Pb_(1-x) Cd_(x) S alloy for example,will exhibit a cadmium-sulfide mole fraction varying radially along itswidth (i.e., between surfaces 202', 204') from a highest value at thecenter (i.e., midway between surfaces 202', 204' on any plane parallelto surfaces 200', 300') to a lowest value at the edges 202', 204'. CurveW7 is an example of the profile of the energy gap, and proportionately,the cadmium-chalcogenide mole fraction, of the layer 14 grown upon acentrally heated substrate. Concomitantly, as the refractive indices ofpure CdS and pure PbS are generally accepted as 2.7 and 4.1 respectivelyin the infrared region, at 300° Kelvin, the refractive index of thelayer varies radially along its width from a lowest value at its centerwhere the CdS mole fraction is highest, to a highest value at thecircumference where the PbS mole fraction is highest. As viewed by light1 incident upon face 200', it is irrelevant whether layer 14 has ahigher index of refraction or is thicker at its center, as in bothinstances, layer 14 acts upon incident light 1 (i.e., light having awavelength greater than that wavelength corresponding to fundamentalabsorption edge of layer 14 at the point of incidence) in the samemanner as a double concave lens.

Conversely, a double convex thin-film lens of the type shown in FIG. 14may be prepared using the circumferential substrate heating apparatusshown in FIG. 13A. The energy gap along a radial parallel to surfaces202', 204' would, as indicated by curve W8, vary from a lowest value atthe center of layer 14 to a highest value at the edges 202', 204'. Theindex of refraction would vary inversely from a highest value at thecenter where the PbS mole fraction is highest to a lowest value at thecircumference where the CdS mole fraction is highest.

Although the substrates 12 are shown in FIGS. 12A and 13A centered withrespect to the heater elements 40, 40' respectively, it is easy toeccentrically position the substrate with respect to the axis of theheater element to prepare a semiconductor having an asymmetricallygraded energy gap along its width. The curve W8 of FIG. 15B is anexample of an asymmetrically graded energy gap. Various combinations ofconvergent and divergent thin-film lenses may be prepared by alternatelyheating the center and then the circumference of a substrate using acombination of the substrate heaters shown in FIGS. 12A and 13A. Inaddition, the energy gap profile of a layer grown upon an unequallyheated substrate may be varied across the thickness as well as acrossthe width of the layer by changing the amount of heat provided by theheater element during growth. Furthermore, by incorporating either thecentral or peripheral substrate heater with a uniform substrate heater40, and independently controlling the heat delivered by each during thegrowth of the layer 14, it is possible to prepare thin film lenses thatare asymmetric (e.g., planoconvex, concavo-convex, plano-concave, orconvexo-concave) as well as symmetric with respect to the thickness ofthe film.

What is claimed, and desired to be secured by Letters Patent of theUnited States, is:
 1. A lens comprising:a layer geometrically describedby the orthogonal dimensions of thickness and width, the layer beingprepared from a lead alloy having a cadmium-chalcogenide mole fraction,the cadmium-chalcogenide mole fraction of the layer depending upon atleast one of the orthogonal dimensions.
 2. The lens set forth in claim 1wherein the mole fraction of the layer varies along the thickness of thelayer.
 3. The lens set forth in claim 2 wherein the mole fraction of thelayer varies uniformly.
 4. The lens set forth in claim 2 wherein themole fraction of the layer varies abruptly.
 5. The lens set forth inclaim 2 wherein one or more abrupt variations in the mole fraction ofthe layer occur along its thickness, each of the variations defining ajunction between adjoining parallel planar regions of constant molefractions.
 6. The lens set forth in claim 5 wherein one of the one ormore abrupt variations comprises a region having a greater mole fractiondisposed between incident radiation and an adjoining region.
 7. The lensset forth in claim 1 wherein the mole fraction of the layer varies alongthe width of the layer.
 8. The lens set forth in claim 7 wherein themole fraction of the layer varies uniformly.
 9. The lens set forth inclaim 8 wherein the layer comprises a diverging lens
 10. The lens setforth in claim 9 wherein the lens comprises a double concave lens. 11.The lens set forth in claim 9 wherein the lens comprises a plano-concavelens.
 12. The lens set forth in claim 9 wherein the lens comprises aconvexo-concave lens.
 13. The lens set forth in claim 8 wherein thelayer comprises a converging lens.
 14. The lens set forth in claim 13wherein the lens comprises a double convex lens.
 15. The lens set forthin claim 13 wherein the lens comprises a plano-convex lens.
 16. The lensset forth in claim 13 wherein the lens comprises a concavo-convex lens.17. A semiconductor device prepared from a lead-cadmium chalcogenidealloy having a first composition, an epitaxial film having a secondcomposition within a range including the first composition depositedupon an insulating substrate in a vacuum of at least 10⁻⁴ Torr,according to a process comprising the steps of:heating the substrate toa temperature that is a function of the second composition and greaterthan 400° C.; maintaining a source of the alloy at a temperature greaterthan its sublimation temperature; and exposing the substrate to thesource until the epitaxial film is deposited on the substrate, the molefraction of cadmium-chalcogenide in said film varying between zero andsix hundredths.
 18. A semiconductor device, comprising:an epitaxiallayer prepared from a source of a lead-cadmium chalcogenide alloy, thesource having a first cadmium-chalcogenide mole fraction, the epitaxiallayer having a second cadmium-chalcogenide mole fraction; the epitaxiallayer having been deposited upon a surface of an insulating substrate ina vacuum of at least 10⁻⁴ Torr; the source having been maintained at atemperature greater than the sublimation temperature of the alloy; thesurface of the substrate having been maintained while the epitaxiallayer was deposited, at an elevated temperature or a sequence ofelevated temperatures indirectly proportional to the second cadmiumchalcogenide mole fraction and greater than 350° C.; and the substratehaving been exposed to the source while maintained at the elevatedtemperature or a sequence of elevated temperatures.
 19. The device setforth in claim 18 wherein the elevated temperature or sequence ofelevated temperatures is greater than 400° C. and the secondcadmium-chalcogenide mole fraction exceeds the first cadium-chalcogenidemole fraction.
 20. The device set forth in claim 18 wherein the surfaceis uniformly maintained at a single temperature.
 21. The device setforth in claim 18 wherein the sequence of elevated temperaturescomprises a uniform variation in temperature while the epitaxial layeris being deposited.
 22. The device set forth in claim 18 wherein thesequence of elevated temperatures comprises an abrupt variation intemperature while the epitaxial layer is being deposited.
 23. The deviceset forth in claim 18 wherein the sequence of elevated temperaturescomprises an abrupt variation in temperature while the epitaxial layeris being deposited and the substrate is being shielded from the source.24. The device set forth in claim 18 wherein the epitaxial layer isdescribed by a plurality of orthogonal dimensions and the value of thesecond cadimum-chalcogenide mole fraction varies along at least one ofthe orthogonal dimensions.
 25. The device set forth in claim 24 whereinthe one of the orthogonal dimensions is the thickness of the epitaxiallayer.
 26. The device set forth in claim 24 wherein the one of theorthogonal dimensions is the width of the epitaxial layer.
 27. Thedevice set forth in claim 18, wherein the epitaxial layer has a pair ofopposed and spaced apart surfaces, and variance in value of the secondcadmiumchalcogenide mole fraction along an imaginary straight linepassing between the pair of surfaces defines a parabola.
 28. The deviceset forth in claim 18, wherein the epitaxial layer has a center spacedbetween opposite edges, and variance in value of the secondcadmiumchalcogenide mole fraction along an imaginary straight linerunning between the opposite edges and passing through the center in aplane parallel to the width describes a parabola.
 29. A semiconductordevice prepared from a lead-cadmium chalcogenide alloy having a firstcadmium-chalcogenide mole fraction, an epitaxial layer having a secondcadmium-chalcogenide mole fraction within a range including the firstmole fraction deposited upon an insulating crystalline substrate in avacuum of at least 10⁻⁴ Torr, according to a process comprising thesteps of:maintaining a source of the alloy at a temperature greater thanits sublimation temperature, said alloy having mole fraction rangelimits of zero and fifteen percent; maintaining the substrate at atemperature within a range of 400° C. to 513° C.; exposing the substrateto the source; and varying the second mole fraction by changing thetemperature of the substrate.